Keep unreferenced blobs newer than prune_grace_period (closes #387)

A backup uploads its blobs before the manifest that lists them, and the
process lock covers one user on one machine, so a prune from another host
or user sharing the destination deleted a running backup's blobs.

Prune now deletes an unreferenced blob only once its listed time is older
than the new config key prune_grace_period (default 24h; a negative value
is refused). A newer blob, or one listed with no time, is kept and counted
(blobs_too_new). The rclone backend takes each object's time from the
listing (use_server_modtime).

Decision: a blob a backup reuses from an earlier run that stopped before its manifest keeps its old time and is not protected; the docs say so.

Model: opus-5-5

internal/storage/conformance_test.go

@@ -8,6 +8,7 @@ import (
 	"reflect"
 	"sort"
 	"testing"
+	"time"
 
 	"sneak.berlin/go/vaultik/internal/storage"
 )
@@ -19,15 +20,17 @@ import (
 // constructor, so the contract is defined once.
 //
 // It exercises the public Storer interface: round-trip, stat, list with
-// prefix filtering, overwrite, delete, delete-of-missing, and not-found on
-// Get and Stat. Each section takes its own fresh backend instance, so the
-// order of sections never matters and no section sees another's objects.
+// prefix filtering, the modification time ListStream reports, overwrite,
+// delete, delete-of-missing, and not-found on Get and Stat. Each section
+// takes its own fresh backend instance, so the order of sections never
+// matters and no section sees another's objects.
 func runStorerConformance(t *testing.T, newStorer func(*testing.T) storage.Storer) {
 	t.Helper()
 
 	conformanceRoundTrip(t, newStorer(t))
 	conformanceOverwrite(t, newStorer(t))
 	conformanceList(t, newStorer(t))
+	conformanceListStreamModTime(t, newStorer(t))
 	conformanceDelete(t, newStorer(t))
 	conformanceNotFound(t, newStorer(t))
 }
@@ -118,6 +121,35 @@ func conformanceList(t *testing.T, s storage.Storer) {
 	}
 }
 
+// conformanceListStreamModTime checks that ListStream gives an object
+// just written a modification time of about now, which prune compares
+// with its grace period.
+func conformanceListStreamModTime(t *testing.T, s storage.Storer) {
+	t.Helper()
+
+	ctx := context.Background()
+
+	err := s.Put(ctx, "blobs/dd/written-now", bytes.NewReader([]byte("data")))
+	if err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	var listed []storage.ObjectInfo
+
+	for info := range s.ListStream(ctx, "blobs/") {
+		listed = append(listed, info)
+	}
+
+	if len(listed) != 1 || listed[0].Err != nil {
+		t.Fatalf("ListStream(\"blobs/\") = %v, want the one object", listed)
+	}
+
+	if time.Since(listed[0].ModTime).Abs() > time.Minute {
+		t.Errorf("ListStream ModTime = %v, want within a minute of now",
+			listed[0].ModTime)
+	}
+}
+
 // conformanceDelete checks that Delete removes an object and that deleting
 // a missing key is not an error.
 func conformanceDelete(t *testing.T, s storage.Storer) {